This invention relates generally to angle headed fasteners, and more particularly concerns the construction of such fasteners to overcome problems and defects stemming from fastener head and shank reception in non-metallic work bores and counterbores.
In the past, the use of angle headed fasteners to connect work panels, as for example aluminum, titanium or composite panels as employed in aircraft structures, has given rise to numerous problems. Among these were the practice of providing countersinks in the work slightly oversize in relation to the outer edges of the received fastener heads. The required removal of such material to produce the oversize countersinks resulted in undesirable notch sensitivity reduced shear bearing area, and decline of fatigue life of the structure. In this regard, most fasteners are designed to withstand shear loading, and in thin sheet applications the shear bearing area is reduced, magnifying the loading per square inch on the bearing area, thereby inducing sooner failure. Knife edge conditions associated with the countersink contribute to the problem. Also corrosion problems developed; for example, protective material filled into annular recesses about the heads tended to work loose during flexing of aircraft structures, so that corrosive fluids could gain access to the work bore along the fastener length, weakening the structure by producing corrosion and stress corrosion. Also, since the fastener head periphery does not completely engage the work bore due to the oversize relation, the development of desirable radial compression and peripheral tension in and within the elastic limits of the work material about the fastener head was not possible.
U.S. Pat. Nos. 3,849,964 and 4,370,081 disclose fasteners overcoming the above problems. Those fasteners incorporate a head having first and second tapered sections that provide interference engagement with the work counterbore, and in addition providing radial compression and peripheral tension in the work.
More recently non-metallic (as for example graphite or boron) work panels have been employed as in aircraft structures, to overcome corrosion problems and to ensure panel composite strength. Such panels may typically incorporate a myriad of graphite, boron or synthetic resin fibers or filaments extending lengthwise in the plane of a panel, representative filament diameters being between 0.0001 and 0.008 inches. Boron strands may for example be coated with silicon carbide, the result being known as "BORSIC". Organic matrices commonly used with boron or graphite filaments are modified epoxy resins, and organic resins such as polyamides and phenolics. Other examples are panels consisting of glass fiber, KEVLAR, polyester or epoxy resin. However, when fasteners are employed to interconnect such panels, and adjacent structure is subjected to high relative shear loading, it is found that load bearing bore surfaces and countersinks (through which the fasteners project) tend to deteriorate as represented by "powdering" of the bore surface material (i.e. the filament ends abrade). This is particularly a problem at more highly shear loaded surfaces, as at the reduced length bore surface which engages the fastener shank immediately fowardly of the fastener head. The problem becomes acute when the panels are subjected to vibratory shear loading, as is common in aircraft. A high interference fit as between the fastener and the non-metallic panel composite tends to cause and increase abrasion during installation, increasing the problem of powdering, i.e. excessive wear and degradation of the bore and countersink/counterbore surfaces in the work.